RESEARCHERS have long been interested in unraveling the secrets of molecular dynamics. Watching a molecule as it evolves over time when exposed to external influences helps reveal how complex biological systems, such as those in the human body, function and change under varying conditions. X-ray crystallography is useful for examining the detailed atomic structure and motion of macromolecules in their crystalline forms. This process has been used for many materials science applications. However, some macromolecules and materials cannot be crystallized. Studying the dynamics of these structures requires a different x-ray diffraction technique that uses faster, shorter, and stronger x rays.

The soft x-ray free-electron laser at Deutsches Elektronen-Synchrotron (DESY) in Hamburg, Germany, known as FLASH, is currently the only laser that can generate the pulses needed for these experiments. An x-ray free-electron laser (XFEL) is a source of intense coherent electromagnetic radiation. It produces femtosecond pulses of 1- to 0.1-nanometer-wavelength light containing trillions of photons. With these fast, short, and powerful beams of light, researchers can obtain high-resolution diffraction patterns and images of noncrystalline structures with nanometer-size features.

In collaboration with the University of California at Davis, Stanford Synchrotron Radiation Laboratory, Uppsala University in Sweden, University of Duisburg-Essen in Germany, and DESY, Livermore researchers demonstrated how XFELs can be used to image the transient, ultrafast dynamics of nanoscale materials and biological structures. The team used FLASH to image the vaporization of a nanometer-size structure etched into a silicon nitride target. With FLASH, the team achieved highly detailed, time-resolved images of the structure as it was destroyed by the laser. This new imaging technique allows researchers to study the noncyclic, or nonrepetitive, phenomena of structures as they undergo violent processes. In addition, researchers can use the technique to better understand the dynamics of condensed matter. The Livermore researchers were funded by the Laboratory Directed Research and Development Program.

In dynamic imaging experiments, a visible-light laser pulse excites a silicon nitride structure before an x-ray free-electron laser (XFEL) is fired on the structure. A multilayer mirror reflects the diffraction pattern from the scattered light onto a charge-coupled device (CCD) camera, while the main light beam travels through the hole in the center
of the mirror.

Faster than a Disintegrating Material
When an electromagnetic pulse hits a noncrystalline structure, light from the pulse scatters, creating a diffraction pattern that characterizes the structure at that specific point in time. The wavelength of light used to create the diffraction pattern determines the size and quality of the features that can be resolved. Prior to FLASH, short-pulse x-ray sources used for imaging either contained an insufficient number of photons to create strong diffraction patterns or produced light at wavelengths that were too long—hundreds of nanometers—to resolve noncrystalline structures measuring only tens of nanometers. Because weak diffraction patterns produce low-resolution images, minute features were blurred and unidentifiable. Non-XFEL pulses also last longer, transferring too much energy to the structure and destroying it before researchers can capture an image.

FLASH produces light at 6- to 60-nanometer wavelengths, making it possible to image features smaller than 100 nanometers. FLASH pulses also contain 1012 photons. Enough of these trillions of tiny light particles are scattered to create clear diffraction patterns of nanometer-size noncrystalline structures. And, because FLASH pulses are only 10 femtoseconds long, researchers can capture an image of a structure before the laser destroys it. According to Livermore physicist Anton Barty, who led the project at DESY, “With these femtosecond pulses, we can break through the radiation damage limit.”

With the x-ray free-electron laser FLASH, researchers produce diffraction patterns of a structure as it undergoes laser vaporization. The patterns show the structure 5 picoseconds before the excitation laser pulse and then as it evolves 10, 15, 20, 40, and 140 picoseconds after the pulse.

Picoseconds in Time
In previous experiments on FLASH in 2006, the team, then led by former Livermore physicist Henry Chapman, who now works at DESY, showed for the first time how femtosecond XFEL pulses could be used to capture static images of a nanometer-size noncrystalline structure. (See S&TR, May 2007, Imaging Complex Biomolecules in a Flash.) These experiments proved that high-resolution images of a noncrystalline material could be acquired before the XFEL pulse destroyed the structure. Because one XFEL pulse contains enough energy to destroy the structure in a single shot, the current team had to develop a method for hitting the structure with a pulse only after it had already begun to disintegrate. The team used a visible-light laser to induce excitation of the structure and then timed the XFEL pulse to fire after the excitation pulse.

For each target, a nanometer-size structure was etched onto a 20-nanometer-thick silicon nitride membrane using a focused-ion beam. The membrane was embedded into and held in place by a silicon wafer. The targets, fabricated at Livermore and shipped to DESY, were produced with low variability.

During the experiments, the XFEL and a visible-light laser were focused onto a micrometer-size spot on the target. First, the system fired the visible-light laser, hitting the target with a 543-nanometer-wavelength pulse lasting 12.5 picoseconds. Shortly after this excitation pulse—at a predetermined time delay—the XFEL fired a 13.5-nanometer-wavelength, 10-femosecond-long pulse. A photodiode connected to the lasers regulated the pulse timing. This process was repeated for multiple targets, with each XFEL pulse firing at a continuously variable delay after the excitation pulse. Thus, each diffraction pattern represents a different point in time during a structure’s breakdown.

A highly reflective multilayer mirror, positioned just behind the structure at a 45-degree angle from the target, separated the diffracted photons from the main XFEL beam. The main beam traveled through a hole in the center of the mirror, while the coherent diffraction pattern was reflected onto a charge-coupled-device camera, which recorded it. An iterative computer algorithm transformed the recorded pattern into an actual image of the structure. With FLASH, the team acquired images with 50-nanometer spatial resolution and 5-picosecond temporal resolution.

Because the team can image a single structure at any point in its evolution over time, they can study nonrepetitive phenomena in violent processes, such as the destruction of a molecule or material. Prior to these experiments, the only way to study a noncrystalline structure’s reaction to external conditions was to take a single shot of multiple samples at the same interval, then average the images together—a technique that blurs any shot-to-shot sample variation. With this new process, the team’s individual snapshots can be sequenced together to create a “movie” of the structure’s evolution over picosecond timescales.

Computer algorithms transform the structure’s diffraction pattern into an actual image. These images are produced from diffraction patterns corresponding to 5 picoseconds before the excitation laser pulse and 10 and 15 picoseconds after the excitation laser pulse.

An Even Brighter Beam
Soon the team will be able to image noncrystalline structures with even smaller features. When the Linac Coherent Light Source (LCLS) comes online this year at Stanford Linear Accelerator Center in Menlo Park, California, it will produce hard x-ray laser pulses with light wavelengths of 1­ to
0.15 nanometers, billions of times brighter than existing x-ray synchrotron sources. LCLS will allow researchers to study materials with finer details at atomic-scale resolutions.

Barty looks forward to the team’s new imaging technique being used in other research applications as well. “These experiments are useful for bioimaging and other Laboratory mission-related areas such as understanding how lasers interact with materials,” says Barty. “They are also important in determining phase transition, crack formation, nucleation, and other material transformations.” With LCLS, just imagine the secrets that will be revealed when the team can study never-before-seen noncrystalline structures on an atomic scale.
—Caryn Meissner